Compact, high-speed wavefront correction devices having deformable mirrors (DMs) are needed for adaptive optics (AOs). In particular, devices are needed to correct wavefront aberrations or distortions, often due to turbulence and/or system optical aberrations. Perhaps the most challenging applications of adaptive optics involve corrections of turbulence on airborne platforms. In this type of environment, a large number of actuators (in excess of 100) are required to actually deform the mirror. A large phase throw (>8 microns) and high, closed-loop bandwidths (typically >1 kHz) are also needed. In addition, the entire adaptive optics system, including the DM wavefront corrector and its controller, must be compact. This requirement drives the optical beam diameter down to 1-2 cm.
Large DM actuator spacing (≧2 mm) is the primary reason for the large size and weight of existing adaptive optics systems. The size of the entire adaptive optics bench tends to vary linearly with actuator spacing. The F-number of the optical system is typically invariant. Therefore, the optical bench size varies linearly with the beam size, and the beam size depends on DM actuator number and spacing. For example, current micromachined membrane and piezoelectrically actuated DMs have actuator spacings of 2 mm and ≧2.5 mm, respectively. An adaptive optics system that relies on these technologies, and provides 37 actuators across the aperture, then requires a 7.4-9.3 cm minimum beam size on the optical bench. In airborne lasercom transceivers, however, the optimum beam size on the optical bench is 1-2 cm. If the number of actuators across the array exceeds about 7-9 (i.e., 37-61 actuators in a hexagonal array), existing DM technologies are simply too big to fit on the lasercom optical bench without requiring a significant increase in bench size and weight.
Conventional bulk micromachined membrane DMs sold today (e.g. FIG. 1) are electrostatically driven using an array of actuators 100 below the membrane 102. A DC voltage is applied to the membrane to deform it into a static parabolic shape (as shown by the dashed lines in FIG. 1), thereby producing tensile stress in the membrane that acts to pull it back into a flat shape. By increasing or decreasing the applied voltage at the metal actuators below the membrane, the membrane is distorted from the parabolic profile, thereby producing the equivalent of local piston stroke.
These membrane DMs suffer from several drawbacks: 1) The membrane is made from silicon nitride, which may undergo dielectric relaxation when DC biased, resulting in short-term drift in the deflection vs. voltage response; 2) Low electrostatic pressure and/or high membrane tensile stress limit the smallest effective actuator pitch (i.e., the spacing between the same edge of adjacent actuators) to 2.0 mm or more; 3) A pre-biased membrane also has limitations in the amplitude of correction at high spatial frequencies (fs=(2*actuator pitch)−1˜0.5 mm−1); 4) To allow room for the membrane to achieve a parabolic shape, the gap “d1” between the membrane and the metal conductors on the backplane is on the order of 40-100 microns; 5) These membrane DMs can achieve only a modest optical phase throw of ˜4 microns, even though they are operated at control voltages of 200-300 V. Electrostatic devices exhibit a quadratic dependence of electrostatic pressure P on the voltage V and gap d, according to the equation:P=∈o(V/d)2 
This relationship says that, all other parameters being equal, a device having a smaller gap will operate at a lower voltage; 6) Due to large gap and high values of membrane residual tensile stress (>100 MPa), the actuator spacing is limited to about 2 mm and significant coupling between actuators is observed; 7) The total number of actuators for a membrane DM is higher than for other technologies because membrane mirrors require additional actuators outside the optical aperture to achieve large deflections at the pupil edge that are necessary to reproduce Zernike polynomials; 8) The minimum actuator spacing dictates the total size of the membrane/actuator array for a given size and number of actuators across the array: 9) Membrane DMs require a second optical element to remove the parabolic curvature from the wavefront; 10) Current membrane DMs are not hermetically sealed and operate in 1 atm air pressure, which strongly dampens membrane oscillations; and 11) They are sensitive to microphonics and electrostatic damage. These features of the conventional membrane DMs, while minimizing costs, significantly reduce membrane dynamic range at high temporal and spatial frequencies.
Hence, there is a need for a compact, high-speed DM to overcome one or more of the drawbacks identified above.